(380a) Kinetic Study of Ammonia Decomposition on Tungsten Carbide for Cleaner Production of Hydrogen from Biomass Gasification

Biomass gasification is one of
the most flexible and energy efficient technologies for production of hydrogen,
syngas, low-to-medium energy fuels and also for power generation. Presence of
impurities such as tars, H₂S, and NH₃ pose major barriers
in the utilization of the biomass gas. When H₂ from this stream is
fed to proton exchange membrane fuel cells, impurity NH₃ molecules
would block the active Pt sites of the anode as well as the acid sites of the
Nafion membrane leading to poor performance. Moreover, when the gasification
stream is used for power generation, undesirable conversion of NH₃
into NO_x through gas turbine combustion can occur with conversion
level as high as 50%. Hence, NH₃ needs to be decomposed to N₂
and H₂ at higher temperatures making NH₃ decomposition an
important reaction to be studied. Ru-based catalysts have proved to be
particularly active and stable for NH₃ decomposition, but are
expensive. Other promising types of less expensive catalysts are carbides and
nitrides of Mo, V, and W. Among carbide materials, tungsten carbide (WC) is of
particular interest as it exhibits catalytic properties similar to those of
platinum [1] as well as has extreme hardness and greater thermal stability.

This paper reports on a kinetic
study of NH₃ decomposition on WC and its comparison with reaction on
tungstated zirconia (WZ) and on commercially available NH₃ synthesis
catalyst (Amomax-10). The effect of the presence of H₂ and CO, the
two main components in syngas, on the behavior of WC is also reported.

The catalysts were characterized
by BET, XRD, SEM, EDX, and temperature programmed reaction (TPRx). The NH₃
decomposition reaction was carried out at 1 atm with temperatures ranging from
475-800°C. The concentration of NH₃ in the inlet stream was 4000
ppm. The time-on-stream experiments showed that WC was characterized by an
induction period which in general decreased in time with increase in
temperature. Complete decomposition of NH₃ was observed at 550°C for
the reaction conditions used. In the presence of syngas components (H₂
and CO), higher temperatures (ca. 750°C) were required for the complete
decomposition of the NH₃. The reasons for this behavior of WC in the
presence and absence of syngas components as well as its power law dependence
will be discussed. A comparison of reaction rate data for WC with that for WZ
and for Amomax-10 will be presented.

1. Levy, R. B., Boudart, M., Science 181 (1973) 547.